Experimental Investigation of the Static-to-Kinetic Friction Transition of Piles in Sand Subjected to Various Loading Rates

The mechanics response and ultimate bearing behavior of piles are significantly influenced by static loading rate. This study investigates the variation in monotonic axial capacity of closed-ended piles subjected to different loading rates. A distinct reduction in total bearing capacity is observed as loading rate increases, primarily attributed to a moderate enhancement in base resistance and a pronounced reduction in shaft resistance. Base resistance is progressively mobilized through densification of sand beneath the pile tip. Simultaneously, fine particles generated by particle breakage form a sandy crust along the shear zone surrounding the shaft. The combined effects of particle crushing, interface shearing, and relative displacement lead to a significant reduction in shaft friction at higher rates, driven by the transition from static to kinetic friction. This behavior is classified in four stages: fully static, nominally stationary, presliding, and rolling. At higher rates, interaction shifts from a sand–steel to sand–sand interface, reducing the friction coefficient, lateral stiffness, and normal stress. A velocity-based friction model, incorporating a backbone curve and capacity prediction formula, is proposed to quantify this transition. The model accurately captures rate-dependent behavior, offering practical value for design and analysis.